Color output devices for color imaging systems, such as color printers, typically represent colors within the context of a device color space. A color space reflects the particular characteristics of the manner in which a device produces a color image. For example, a CRT monitor utilizes red, blue and green phosphors to generate an image, and thereby operates within a red-green-blue (RGB) color space. In contrast, a color printer may utilize cyan, magenta, and yellow inks or toners to generate images, and thereby operates within a cyan-magenta-yellow (CMY) color space.
The gamut of an imaging device is defined as the fill range of colors that the device can reliably reproduce within its color space. In practice, a device's gamut is limited by characteristics of the methods that the device uses to produce colors, and is almost always a subset of the visible spectrum. For example, most color laser printers cannot reproduce saturated colors as bright as those which appear on a computer monitor. In some devices, there are colors which are visible in the device gamut but which produce undesirable artifacts. Typically, these artifacts occur at the boundaries of the device gamut, e.g. in areas of high saturation. For example, a color laser printer may represent 100% blue as a combination of 100% cyan toner and 100% magenta toner, where the percentage refers to the saturation, or intensity, of the toner that is placed on the paper. When such high saturation values are called for, the result may “smudge” or “bloom” at the edges of the objects.
Blooming is one particular example of a phenomenon that becomes pronounced when objects are printed with highly saturated secondary or tertiary colors. In a printer that operates with a CMY color space, for example, the secondary colors of concern are red, blue and green, which are each formed by combining two of the component colors, cyan, magenta and yellow. It has been observed that, when two primary colors are successively deposited in an area of an image to produce a secondary color, the latter one of the two primary colors to be deposited ends up covering an area larger than the actual area of the object. For example, red text is generated by depositing magenta toner and then yellow toner. When the resulting image is examined, the yellow toner forms a “halo” around the edges of the character which produces a smudged appearance. In addition, the actual color of the character may have an orangish hue, rather than being red.
In the past, various methods have been employed to limit and/or modify the gamut of an imaging device, in an effort to reduce these artifacts. For example, a tonal reproduction curve (TRC) is commonly used to redistribute the nominal color space within a device gamut. These curves basically comprise transfer functions that are individually applied to the different color components of the device's color space. For example, in a CMY printer, a nominal color value (c,m,y) is adjusted by the toner reproduction curve to generate an adjusted color value (c′,m′,y′). The computation of each adjusted component of the color value is a function of only its own nominal color value component. In other words, c′ is a function of c, m′ is a function of m, and y′ is a function of y. As such, tonal reproduction curves cannot be used to correct for artifacts which result from combinations of color components.
A simple approach that has been used to restrict the gamut of an image device is to reduce all color values by a global modification factor. For example, in a CMY printer where at least 90% of the gamut is artifact-free, a modified color value (c′,m′,y′) can be computed from a nominal color value (c,m,y) as:c′=0.9cm′=0.9my′=0.9yThis method results in a restriction of the entire device gamut, including areas were restriction may not be necessary. In addition, it causes every color to be modified, even those which were originally within the artifact-free portion of the gamut.
Another method that has been employed in the past is to clip values which lie outside of the artifact-free portion of the gamut, i.e., to limit them to a maximum value. For example, in a CMY printer where the maximum cyan, magenta and yellow values in the artifact-free portion of the gamut are represented as cmax, mmax and ymax, the modifying color value (c′,m′,y′) is computed from the nominal color value (c,m,y) as:c′=min(c,cmax)m′=min(m,mmax)y′=min(y,ymax)This approach also results in undesirable visual side effects. For example, if cmax is 95% cyan, all cyan values in the nominal device space which are greater than 95% cyan are forced to the single value of 95% cyan in the artifact-free device space. Thus, in an image having a gradient in which cyan progresses from 0% to 100%, the clipping of all cyan values greater than 95% will be readily apparent in the image.
It is desirable, therefore, to provide a method by which device-dependent image quality artifacts can be reduced or eliminated without adversely affecting the quality of images resulting therefrom.